Storage-stable aqueous suspensions of pesticides (flowables) to be diluted with water have been used to an increasing extent during recent years. Several environmental advantages are associated with products of that kind, e.g. in comparison to dispersible powder preparations there are no problems with dust and in comparison to emulsion concentrates in which the presticide is dissolved in a solvent which is emulsified in water by means of surfactants, it is possible to avoid the use of toxic and disagreeable solvents.
Another known type of pesticidal concentrate is an oil-containing pesticide suspension in which the oil is emulsifiable in water. When such a composition is diluted in water for the final use the emulsifier must be able to distribute the oil without forming gels or lumps and to distribute the pesticide in the aqueous phase. It is important that the distribution in the aqueous phase takes place quickly and without difficulties. As most field-spray equipments are nowadays equipped with effective impellers it is acceptable it is acceptable that a minor separation of oil or pesticide takes place on standing, but it is not acceptable if the oil gradually separates in such a manner that the pesticide is agglomerated into greasy layers or precipitates. The emulsifier used should also ensure that both oil and pesticide are distributed efficiently on the plants.
On standing most suspension concentrates normally tend to separate out so that a clear upper layer appears. Normally, this has no practical consequence, but if the suspension comprises both oil and water in one or two phases, both of these phases as well as the suspended phase of solid particles should be able to be shaken into a homogeneous slurry which will not separate immediately into separate fractions.
It is decisive that the surfactant composition in oil-containing compositions is active under different conditions. It is especially difficult to optimize the surfactant system if the amount of emulsifier is low and if the liquid phase is non-aqueous or if the water content is low, e.g. below 10%, especially below 3-5%, by weight. In these circumstances it may be difficult to distribute the oil-containing suspension in the aqueous phase without the forming of greasy precipitations comprising both oil and pesticide particles.
Oil-containing pesticidal dispersions can be advantageous compared to aqueous systems when the oil acts as activator in relation to the biological effect. A non-aqueous system or a system with a low water content will be preferred under circumstances in which water acts detrimentally on the stability, e.g., when water has an adverse effect on the chemical stability, promotes the development of toxic decomposition products and/or dissolves the finely suspended pesticidal active components.
Applicants' co-pending European application EP, A2, 142670 describes stabilized liquid herbicidal compositions comprising, as active ingredient, a herbicidally effective amount of carbamates such as phenmedipham, optionally in admixture with metamitron, in which the active ingredient in a finely ground state is dispersed in a liquid phase which comprises at least one oily component in an amount of 5-75% by weight, and at least one surfactant in an amount of 5-60% by weight, and optionally water. Example 15 of said patent application describes how solid, finely ground phenmedipham is suspended in either mineral oil-containing micro-emulsions or mixtures of organic solvents and mineral oil which in dilution with water form micro-emulsions. The concentration of the finely ground pesticide is decisive for the rate of dissolution. When this is sufficiently low, e.g. below 150 g/l, it is normally easier to mix the composition with water than when the concentration is higher which is seen by the fact that generally only a small separation of pesticide takes place when water is added.
Biological testing of phenmedipham-comprising compositions has shown that it is important that the content of oil is sufficiently high in order to achieve the biological effects of the oil-containing suspension concentrates when the tests are carried out not only in greenhouses but in field trials. Based on biological testing, it has been found that the liquid phase should preferably consist of oil and surfactant which activate the biological effect, i.e. the weed killing effect is increased when oil and/or surfactant is added to the composition.
When the pesticide is a herbicide and this herbicide is a mixture of phenmedipham and metamitron in the form of a suspension it is advisable that the content of mineral oil is high and that the content of water, organic solvents and surfactants is low as metamitron is much less soluble in mineral oil than in water and surfactant. A high solubility will normally lead to a physically unstable suspension.
Nowadays, it is normal practice for farmers to admix metamitron and phenmedipham compositions with water in tanks. Metamitron is normally marketed as a water dispersable powder preparation containing 70% by weight of metamitron, and phenmedipham is normally marketed as an emulsifiable preparation based on isophorone and with a content of active ingredient of 160 g/l. By mixing in tanks the degree of distribution of the products in the aqueous phase is normally decreased. As the phenmedipham-containing preparation containing emulsifier is preferably used in concentrations of about 1% by weight, a fast up-take of isophorone in the dilution water will take place and this will lead to crystallization of phenmedipham. It appears that the crystallization will take place on the suspended metamitron particles. In practice, this conversion can be seen by the fact that the slurried particles tend to sediment, and depending on the surfactant composition in the phenmedipham preparation there will also be a tendency to some flocculation.
Biological testing on weeds in beets has shown that oil-containing suspension concentrates comprising phenmedipham and metamitron have a surprisingly strong herbicidal effect if the active components are efficiently distributed in the aqueous phase. This must be due to the fact that then, there is no possibility for undesirable interactions in the dilution step, but the invention is not limited to this theory.
Metamitron is normally used in larger amounts than phenmedipham. Therefore, it is advisable that the concentration of metamitron in mixed preparations is 3-5 times as high as the concentration of phenmedipham. Therefore, the dosage per hectare will be higher, and compared to a preparation comprising only phenmedipham a larger volume of product must be used. In order that the amount spread on the fields is not too high it is important that the concentration of the active components is as high as possible. In the Examples in EP, A2, 142670 dealing with phenmedipham and metamitron in non-aqueous oil-containing suspensions, the total content of pesticide is about 250 g/l. It has been found that it would be advantageous to increase this amount so that the total amount of pesticide is at least 280 g/l, more preferably at least 300 g/l, e.g. 320 g/l. By using high concentrations it is possible to spread a lower amount of the final pesticidal composition on the fields, thus giving a benefit in application.
A high content of oil will increase the toxic influence on the plants. In compositions comprising metamitron a high content of water will normally act detrimentally on the storage stability. Hence, it is not possible to substitute a major part of the water for oil.
In the Examples of EP, A2, 142670 all surfactant compositions are combinations of ionic and non-ionic surfactants. In the Examples of the above-mentioned European patent application the main part of the ionic component(s) consists of anionic surfactants (dodecylbenzene sulphonic acid and esters of phosphoric acid) partially neutralized with a base. With surfactants of this type it has not been possible to prepare suspension compositions comprising 260 g/l of metamitron and 60 g/l of phenmedipham when the amount of oil is high, e.g. above 50-55% by weight, and the total amount of surfactant is low, e.g. below 10% by weight. In EP, A2, 142670 it is mentioned that ampholytes can be used as surfactants in oil-containing suspensions of phenmedipham. However, only 3% of the total amount of surfactant is a coco alkylamino propionic acid, and the amount of this compound constitutes only a minor amount, i.e. about 6% by weight of the total amount of ionic surfactants.
CH P 647929 (Schafer et al.) describes pesticide compositions in which the active component is dissolved in an aqueous phase comprising at least one non-ionic surfactant and a ionic surfactant wherein the non-ionic surfactant has a HLB-value of 5-20 and the ionic surfactant component is an ampholyte, optionally in combination with an anionic or cationic surfactant. There is no mentioning of the use of oily substances.